1. Field of the Invention
The present invention relates generally to an aircraft, and more particularly to enhanced flight control systems and methods for a jet powered tri-mode aircraft.
2. Description of the Related Art
U.S. Pat. No. 5,951,608 to Osder, entitled “Flight Control System For Jet Powered Tri-Mode Aircraft,” which is assigned to the assignee of the present invention, discloses a basic flight control system for a jet-powered tri-mode aircraft. The disclosure of the '608 patent is incorporated by reference herein. The jet-powered tri-mode aircraft generally has three primary modes of operation, which are a helicopter mode, a compound mode, and a fixed-wing mode. The jet-powered tri-mode aircraft is designed to take off vertically in helicopter mode and then to fly horizontally in fixed-wing mode.
In the helicopter mode, power is applied to a rotor blade of the aircraft so that the aircraft can take off in a vertical direction. The rotating rotor blade provides the advantage of small space takeoffs and landings. In the helicopter mode, the forward speed of the aircraft is controlled by tilting the rotor generated thrust vector forward or aft. Initially, most of the available power is applied to the rotor blade, and the rotor blade provides most of the aircraft's maneuverability and control. Control of the aircraft is augmented by the aero surfaces (e.g., the elevons and horizontal tail surfaces) due to the rotor downwash enhancing the effectiveness of the aero surfaces at low traveling velocities. The rotor tip jets eliminate the need for an anti-torque system such as a tail rotor or equivalent. However, differential left and right thrusters are used to obtain yaw control. These and other features are disclosed in detail in the '608 patent.
In the compound mode, the rotor blade is gradually unloaded until it provides essentially zero lift. Complete unloading occurs at a predetermined velocity, sometimes referred to as the conversion speed. In the compound mode, the forward speed of the aircraft is maintained by applying power to the conventional jet engine to direct the engine exhaust in the rear direction. The aero surfaces provide most of the aircraft's maneuverability and control at higher speeds of the compound mode. The rotor swashplate controls are blended with the elevons, the rudder, and the horizontal tail aerodynamic controls, with the aerodynamic surface controls becoming dominant at the higher speeds while the rotor swashplate controls have their gains gradually reduced to near zero at higher speeds of the compound mode. The canard is also articulated, so that combined with the horizontal tail, these aero surfaces provide all of the aircraft's lift at the conversion speed. Once all of the aircraft's weight is supported by the lift generated by the aero surfaces, the rotor blade is rapidly stopped and locked in place and the aircraft operates in the fixed-wing mode. Further details regarding the technique to stop and re-start the rotor blade are disclosed in, for example, U.S. Pat. No. 6,193,464 to Nyhus and Osder, entitled “Active Brake Control For Rotor/Wing Aircraft,” which is assigned to the assignee of the present invention.
In the fixed-wing mode, the aero surfaces and the locked rotor blade provide all of the aircraft's lift. The canard and the horizontal tail positions are aligned to allow the aircraft to more efficiently travel at high speeds. To control the aircraft, the basic aircraft stabilization and control concept is implemented and is based on commanding the desired aircraft velocity vector. The velocity vector control is used to provide the integrated flight path and rotor speed control of the aircraft in all of the flight modes. The basic aircraft stabilization and control concept using the desired aircraft velocity vector is disclosed in, for example, U.S. Pat. No. 5,001,646 to Caldwell and Osder, entitled “Automated Helicopter Flight Control System,” which is assigned to the assignee of the present invention.
To integrate the propulsion control and the flight control of the aircraft in the compound mode, the '608 patent disclosed a method of controlling rotor speed via modulation of the collective position. In addition, the '608 patent disclosed a diverter valve to split the engine exhaust between the rotor tip jets and the cruise nozzle. When the aircraft is hovering and at low helicopter speeds, all the engine exhaust flows to the rotor tip jets. As the aircraft moves through the compound mode, the diverter valve decreases flow to the rotor tip jets and increases flow to the cruise nozzle. Once the aircraft has completely transitioned to the fixed-wing mode, the diverter valve is closed to the rotor tip jets and is open to the cruise nozzle.
One drawback of the '608 patent relates to the diverter valve being unable to adequately exit the engine exhaust during the compound mode causing the engine to experience a choke condition, thus resulting in the engine stalling. The problem is fatally intolerable for an aircraft. Another drawback of the '608 patent is the difficulty in determining and maintaining the flight envelope for jet-powered tri-mode aircraft. This is because the flight envelope for multi-flight mode aircrafts is different depending on the flight mode. For example, the flight path and the maximum aircraft speed differ when in the helicopter mode compared to the fixed-wing mode. Also, when in the compound mode, the flight envelope will be different depending on the extent of the unloading of the rotor blade.